Re: [RFC PATCH 15/26] x86/alternatives: Non-emulated text poking

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Tue, Apr 07, 2020 at 10:03:12PM -0700, Ankur Arora wrote:
> +static int __maybe_unused text_poke_late(patch_worker_t worker, void *stage)
> +{
> +     int ret;
> +
> +     lockdep_assert_held(&text_mutex);
> +
> +     if (system_state != SYSTEM_RUNNING)
> +             return -EINVAL;
> +
> +     text_poke_state.stage = stage;
> +     text_poke_state.num_acks = cpumask_weight(cpu_online_mask);
> +     text_poke_state.head = &alt_modules;
> +
> +     text_poke_state.patch_worker = worker;
> +     text_poke_state.state = PATCH_SYNC_DONE; /* Start state */
> +     text_poke_state.primary_cpu = smp_processor_id();
> +
> +     /*
> +      * Run the worker on all online CPUs. Don't need to do anything
> +      * for offline CPUs as they come back online with a clean cache.
> +      */
> +     ret = stop_machine(patch_worker, &text_poke_state, cpu_online_mask);

This.. that on its own is almost a reason to NAK the entire thing. We're
all working very hard to get rid of stop_machine() and you're adding
one.

Worse, stop_machine() is notoriously crap on over-committed virt, the
exact scenario where you want it.

> +
> +     return ret;
> +}